Thyrotropin-Releasing Hormone Stimulates Nitric Oxide Release from GH3 Cells

Constitutive nitric oxide synthase (NOS) is expressed in the rat adenohypophysis but the mechanisms regulating its activity at the cellular level remain to be elucidated. The effect of TRH on nitric oxide release from GH3 cells was studied by means of reverse-phase HPLC to measure NO−2 and NO−3 concentrations in the incubation medium, and by polarography using electrodes specific for NO. Medium NO−2 concentrations in the incubation medium were dependent on the incubation time, and were further increased by sodium nitroprusside (SNP) or high potassium. NO−3 was detectable only in the presence of 100 μM SNP. Addition of l-arginine increased medium NO−2 concentrations. Diamino-hydroxypyrimidine decreased medium NO−2 concentrations, which were restored by the addition of (6R)-5, 6, 7, 8-tetrahydro- l-biopterin (THB). TRH elicited dose-related increases in medium NO−2 concentrations and in nitric oxide-specific currents, which were abolished by Nω-nitro- l-arginine methyl ester. TRH failed to increase medium NO−2 concentrations in cells loaded with an intracellular Ca2+-chelating agent. The findings suggest that mobilization of intracellular Ca2+ by TRH stimulation activates Ca2+-dependent NOS in GH3 cells.